Also known as tertiary recovery. In oil and gas production, techniques used to extract crude oil that cannot be effectively produced by primary or secondary recovery methods. EOR involves injecting carbon dioxide (CO₂) or other gases, steam, or chemicals into an oil-bearing geologic formation to raise reservoir pressures and increase the mobility of oil. EOR can substantially increase the percentage of oil recovered from a field.

EOR differs from secondary recovery in that secondary recovery uses injection to physically displace and push oil to producing wells, while EOR uses injection to change the characteristics of oil and reservoir fluids so that they flow more easily.

EOR projects may be performed for dual purposes of oil recovery and carbon sequestration. For more information on carbon sequestration projects involving EOR, see Practice Note, Regulatory and Real Property Considerations for Carbon Capture and Sequestration (CCS) Projects: EOR Projects.